Don't redeclare environ if std headers already did

This would cause a redundant redeclaration warning on some versions of
Linux otherwise.

src/common/compat.c

@@ -16,7 +16,11 @@
  * We also need it to make memmem get defined (where available)
  */
 /* XXXX023 We should just use AC_USE_SYSTEM_EXTENSIONS in our autoconf,
- * and get this (and other important stuff!) automatically */
+ * and get this (and other important stuff!) automatically. Once we do that,
+ * make sure to also change the extern char **environ detection in
+ * configure.in, because whether that is declared or not depends on whether
+ * we have _GNU_SOURCE defined! Maybe that means that once we take this out,
+ * we can also take out the configure check. */
 #define _GNU_SOURCE
 
 #include "compat.h"
@@ -1663,9 +1667,11 @@ make_path_absolute(char *fname)
 }
 
 #ifndef HAVE__NSGETENVIRON
-/* FreeBSD needs this; it doesn't seem to hurt other platforms. */
+#ifndef HAVE_EXTERN_ENVIRON_DECLARED__
+/* Some platforms declare environ under some circumstances, others don't. */
 extern char **environ;
 #endif
+#endif
 
 /** Return the current environment. This is a portable replacement for
  * 'environ'. */